Natural landscapes, certain agricultural uses, and recently fallowed farmland were associated with differences in reported coccidioidomycosis incidence in California, according to a population-based study published in Science Advances.
Coccidioidomycosis, also known as Valley fever, is caused by inhaling airborne spores of Coccidioides fungi. Its symptoms can overlap with those of other respiratory infections, making laboratory testing important for distinguishing the disease from other causes of community-acquired pneumonia.
Researchers analyzed 65,657 confirmed cases reported to the California Department of Public Health between April 2008 and March 2021. Cases were linked by residential address or ZIP code to annual maps of natural vegetation, agricultural activity, and fallowed land.
The analysis covered 20 California counties with an average annual incidence above five cases per 100,000 patients and at least 10 cases per study year. Researchers divided the region into hexagonal areas of approximately 140 square kilometers and compared monthly incidence with the amount of each land-cover type.
Reported cases increased from 1,708 during the 2008 study year to a peak of 9,778 in 2017. Incidence was highest in Kern County and generally peaked during the fall.
Larger areas of shrubland, barren ground, and grassland were associated with higher coccidioidomycosis incidence. For every standard-deviation increase in the area covered, incidence was 62 percent higher for shrubland, 34 percent higher for barren land, and 18 percent higher for grassland.
Associations varied across agricultural settings. Greater coverage of grain and hay, field crops, corn, and cotton was linked to higher incidence. In contrast, deciduous fruit and nut orchards, truck crops and berries, and rice fields were associated with lower incidence.
Total fallowed land was not associated with incidence when considered as a single category. However, newly fallowed land and land fallowed at any point during the previous 4 years were linked to higher incidence. The relationship also depended on how the land had previously been cultivated. Newly fallowed fields with a mixed cropping history were associated with higher incidence, whereas some land previously used for deciduous fruits, nuts, or field crops was associated with lower incidence.
The researchers suggested that differences in soil conditions, vegetation, dust production, and agricultural disturbance could help explain the findings. However, the study did not establish whether Coccidioides was present in the agricultural soils or dust.
The findings identify environmental and seasonal contexts that may support consideration of coccidioidomycosis testing in patients with compatible respiratory symptoms. The study did not assess the performance of any diagnostic method.
The analysis was observational and could not establish that particular land uses caused infections. Cases were assigned according to patients’ residences and diagnosis dates, which may not represent the location or timing of exposure. Surveillance data may also underrepresent agricultural workers with limited access to health care.
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